XCX. P~tentialReaction between Sodium-Potassium Alloy and Teflone in a Gas-Circulation System
reaction was received on December 1, THIS JOURNAL asked about our interest in including laboratory accident reports in the column. Several dozen reports of laboratory accidents and fires have been pub-
T. Kimura and G. R. Freeman, Chemistry Department. University of Alberta, Edmonton, Alta. T6G 2G2
While attempting to purify propane to the degree that would allow electron mobilities to be measured in the liquid, a n unexpected type of accident occurred. The purpose of this note is to briefly describe the accident and to state how it can he avoided. Liquid hydrocarbons in this laboratory and elsewhere are sometimes freed of electron scavengers (oxygen, peroxides, conjugated diolefins, etc.) by stirring them for several hours with sodium-potassium alloy, which is a liquid a t room temperature. The treatment is done in a normal vacuum system, the hydrocarbon having been previously treated in adjacent sections of the system with potassium hydroxide pellets to remove carbon dioxide, then lithium aluminum hydride to remove water. Each treatment-bulb contains a
Teflon %sated magnetic stirring bar that is used to agitate the heterogeneous mixture of hydrocarbon liquid and purification reagent. Propane could not be purified in the same system because the sodium-potassium alloy freezes at -11°C. whereas pmpane could not be handled conveniently as a liquid a t temperatures much above its normal boiling point ( - 4 2 C ) . Propane gas at 2 atm pressure was therefore circulated over the surface of the alloy in a trap at room temperature. A rotating-vane type of gas circulator drew propane from a 3 liter reservoir, pushed it over the alloy and hack into the reservoir. The alloy was stirred gently to provide a fresh surface. The treatment proceeded overnight with(Continued onpogeA89)
To provide laboratory fire and accident information as a regular feature in this column we are seeking reports that have not been published before, as well as reports that have had very limited circulation. Reports of an emergency nature, such as fatalities from experimental electraphoresis or laser apparatus, should be reported immediately after the facts are determined as a letter to a journal with a weekly circulation or one that is mast likely to reach everyone who needs t o know. In the next few months in this column we plan to publish information on accident investigation and cause analysis, the new uniform fire incident report, and the origin and development of the NFPA Manual of Hazardous Chemical Reactions. Reliable data on laboratory accidents and fires is needed not only as a basis for education, but also for the development of national standards for laboratory safety.
Volume 50. Number 2. February 1973
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(Continuedfrom pogeA85) out incident. The next morning the speed of the stirrer was increased. Immediately, a red glow formed around one of the ends of the stirring bar, which was beneath the surface of the alloy and touching the glass wall of the trap. The stirring motor was shut off and the area of the glow increased. Within a few seconds the glass wall a t the origin of the "red spot" melted: the pressure in the system blew a bubble in the wall and broke it, the resulting hole having a 1 cm diameter and fused edges. Some of the alloy was blown through the hole; it burned where it landed. There was alsa a yellow flame at the hole for shout 30 seconds: a large amount of carbon black formed inside the system. The reaction might have been initiated by triboelectric discharges between the allay and glass (I), through the gas bubble around the end of the stirrer where it touched the wall. Although chemists have not paid much attention to triboeleetric processes ( I ) , atmospheric lightning is presumably an example of such phenomena. Highly reactive ions and radicals are produced in electric discharges. In the present system they probably attacked the adjacent Teflonmon the stirring bar. Friction between the Teflon* and the wall might alsa have generated heat that contributed to the ultimate ignition. Decomposition of the plastic gave products, perhaps C ~ I . that reacted rapidly with the alkali metals and generated more heat, and so on, at a quickly accelerating rate. The red glow was probably due to luminescence from potassium atoms excited by the reaction (cherniluminescence), and initially perhaps also by the triboelectric discharge. To avoid the reoccurrence of this mishap, gases will he purified by circulation aver a large film of alkali metal, such as a sodium mirror, rather than over a liquid alloy. The triboeleetric discharges between the liquid metal and the glass, through the gas, would be themselves have generated impurities in the propane, thereby redueing thevalue of the procedure. It might be a worthwhile general precaution not to use Teflon"-coated stirrers to agitate liquid alkali metal alloys in gas systems. Triboelectric discharge does not occur when liquid hydrocarbons are stirred with the liquid alloy because the breakdown potential of a liquid hydrocarbon (-10" V/cm) is about two orders of magnitude greater than that of a gas at 2 atm presSUTB.
REFERENCE ( I ) S. De Paoli and 0. P. Strausz, Con. J Chern., 48, 3756 (1970). and references therein.
(A similar reaction was reported in a 1966 communication by R. H. Scott, Battelle-Northwest, Richland, Washington in the "Manual of Hazardous Chemical Reactions," 4th Edition, published in 1971 by the National Fire Protection Association, 60 Batterymarch St., Boston, Mass. 02110. The report stated "Sealing tape made of Teflon ' (polytetraflunroethylene or polyhexafluoroethylene) burned vigorously in contact with sodium-potassium alloy in a helium atmosphere.") Volume 50. Number 2. February 1973
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